如何使字符串类型可为空

I want to make the Middle Name of person optional. I have been using C#.net code first approach. For integer data type its easy just by using "?" operator to make in nullable. I am looking for a way to make my sting variable nullable. I tried to search but could not find the way to make it nullable.

我想让人的中间名可选。我一直在使用C＃.net代码的第一种方法。对于整数数据类型，只需使用“？”即可轻松实现运算符可以为空。我正在寻找一种让我的sting变量可以为空的方法。我试图搜索但找不到让它可以为空的方法。

Below is my code. Please suggest me how to make it nullable.

以下是我的代码。请建议我如何让它可以为空。

public class ChildrenInfo
{
    [Key]
    public int ChidrenID { get; set; }

    [Required]
    [Display(Name ="First Name")]
    [StringLength(50,ErrorMessage ="First Name cannot exceed more than 50 characters")]
    [RegularExpression(@"^[A-Z]+[a-z]*$",ErrorMessage ="Name cannot have special character,numbers or space")]
    [Column("FName")]
    public string CFName { get; set; }

    [Display(Name ="Middle Name")]
    [RegularExpression(@"^[A-Z]+[a-z]*$",ErrorMessage ="Middle Name cannot have special character,numbers or space")]
    [StringLength(35,ErrorMessage ="Middle Name cannot have more than 35 characters")]
    [Column("MName")]
    public string? CMName { get; set; }





}

7 个解决方案

#1

String is a reference type and always nullable, you don't need to do anything special. Specifying that a type is nullable is necessary only for value types.

String是一个引用类型，并且始终可以为空，您不需要做任何特殊操作。只有值类型才需要指定类型可为空。

#2

Strings are nullable in C# anyway because they are reference types. You can just use public string CMName { get; set; } and you'll be able to set it to null.

无论如何，字符串在C＃中都可以为空，因为它们是引用类型。你可以使用公共字符串CMName {get;组;并且你将能够将其设置为null。

#3

System.String is a reference type so you don't need to do anything like

System.String是一个引用类型，因此您不需要执行任何操作

Nullable<string>

It already has a null value (the null reference):

它已经有一个空值（空引用）：

string x = null; // No problems here

#4

It's not possible to make reference types Nullable. Only value types can be used in a Nullable structure. Appending a question mark to a value type name makes it nullable. These two lines are the same:

无法使引用类型为Nullable。只有值类型可以在Nullable结构中使用。将问号添加到值类型名称使其可为空。这两行是相同的：

int? a = null;
Nullable<int> a = null;

#5

string type is a reference type, therefore it is nullable by default. You can only use Nullable<T> with value types.

string类型是引用类型，因此默认情况下它是可为空的。您只能将Nullable 与值类型一起使用。

public struct Nullable<T> where T : struct

Which means that whatever type is replaced for the generic parameter, it must be a value type.

这意味着无论为泛型参数替换何种类型，它都必须是值类型。

#6

As others have pointed out, string is always nullable in C#. I suspect you are asking the question because you are not able to leave the middle name as null or blank? I suspect the problem is with your validation attributes, most likely the RegEx. I'm not able to fully parse RegEx in my head but I think your RegEx insists on the first character being present. I could be wrong - RegEx is hard. In any case, try commenting out your validation attributes and see if it works, then add them back in one at a time.

正如其他人所指出的那样，字符串在C＃中总是可以为空的。我怀疑你是在问这个问题因为你不能把中间名留空或空白？我怀疑问题在于您的验证属性，很可能是RegEx。我无法完全解析RegEx，但我认为你的RegEx坚持第一个角色出现。我错了 - RegEx很难。在任何情况下，尝试注释掉您的验证属性并查看它是否有效，然后一次一个地添加它们。

#7

You don't need to do nothing, the modelbinding will pass null to variable without problems.

您不需要做任何事情，modelbinding会将null传递给变量而不会出现问题。

#1